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**sdesic** · 05-29-2013, 03:25 PM

Two scope screenshots TX signal

Difference is time scale

**sdesic** · 05-29-2013, 03:34 PM

Originally posted by Davor View Post

You must find points with low impedance to avoid loading your device with a long cable and low impedance.

Start with Tx, but make a 1:10 or so attenuator, just reverse the resistors as seen in http://www.ledametrix.com/oscope/

Next connect a scope (with 1:1 probe) to output of preamp, and re-do coil nulling.

The most important thing is to supply your device with batteries, as a PC PSUs are not famous for their isolation.

I have made fist one with potentiometer 50k and one capacitor 100uF 16V
Working on battery power.

Špansko selo je kod mene - preamp output ? It's easy to say if you know

**Davor** · 05-29-2013, 03:52 PM

Preamp, or preamplifier is a thing that follows after a RX coil. U101a, pin7 is the preamp output. The best point to stick your soundcard probe is at a resistor R27, as it is firmly on DC ground potential, and it's upper end has only AC component on it.

BTW, I might attend geokamp in mid June, see http://www.kresevski-citrin.com/inde...d/14-4-geokamp
Could be interesting

**sdesic** · 05-29-2013, 05:27 PM

Made new nulling and got RX coil to get 2.1mV from starting 150mV
BUT after that I can't stop detector from going beep beep ... 4x second
if I flex coil it stops little and starts in few seconds again... pots are not responsive

LF353 PIN7 soundcard probe capture

geocamp close to former gold mine... nice

**sdesic** · 05-29-2013, 05:46 PM

Moved coils from that nulling point an it is working Ok again ... less depth but working

**porkluvr** · 05-29-2013, 08:03 PM

Originally posted by sdesic View Post

Moved coils from that nulling point an it is working Ok again ... less depth but working

From looking at your oscilloscope photos in #1126, you have a bad problem either with your test apparatus/technique and/or the transmit section has serious issues.

Don't fool yourself into thinking #1126 is OK. That looks like nothing but noise.
Fix your transmitter, and then you can worry about null.

**sdesic** · 05-30-2013, 04:29 AM

I don't know if it is my audio card oscilloscope or it is problem with MD.
I will try second probe - with resistors only and post screenshot again

**sdesic** · 05-30-2013, 08:10 AM

This is screen capture - sound card oscilloscope with 100k and 22k resistors

connected parallel to TX coil

and with nothing connected to it

**sdesic** · 05-30-2013, 08:58 AM

Scope with pot and one capacitor - with short cable gave good results without signal connected

with connection parallel to TX - screenshot

**sdesic** · 05-30-2013, 09:02 AM

and RX on pin 7 of preamp and ground

**sdesic** · 05-30-2013, 09:06 AM

same pic ... sorry about that
here it is again

**Davor** · 05-30-2013, 11:32 AM

You are mostly interested in envelope of the signal, so you may increase a time base to 10ms, or even 100ms and it will give you a somewhat less confusing display. This is a nice display of a signal sampled near Nyquist frequency, and there is nothing you can do to make it better with the existing soundcard. It would improve if your sampling rate is 96k, but it is unlikely that your soundcard supports it.